Light emitting nitride semiconductor device, and light emitting apparatus and pickup device using the same

ABSTRACT

There is provided a light emitting device having high luminous efficacy or emission intensity. The device includes a light emitting layer provided between n- and p-type layers of nitride semiconductor formed on a GaN substrate. The light emitting layer is formed of a well layer or a combination of well and barrier layers. The well layer is made of a nitride semiconductor containing an element X, N and Ga, wherein X is As, P or Sb. The ratio of the number of the atoms of element X to the sum of the number of the atoms of element X and N, is not more than 30 atomic percent. The well layer contains Mg, Be, Zn, Cd, C, Si, Ge, Sn, O, S, Se or Te as an impurity for improving the crystallinity of the well layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to light emitting nitridesemiconductor devices having high luminous efficacy, and light emittingapparatus and optical pickup devices employing the same.

[0003] 2. Description of the Background Art

[0004] Japanese Patent Laying-Open No. 8-64870 discloses a lightemitting device having a stack of gallium nitride based compoundsemiconductor layers, wherein an active layer is formed of a galliumnitride based compound semiconductor in which the nitrogen is partiallyreplaced with phosphorus and/or arsenic and the active layer is dopedwith at least one dopant selected from the group consisting of Mg, Zn,Cd, Be, Ca, Mn, Si, Se, S, Ge and Te. In this prior art, the activelayer is doped for the purpose of making the emission wavelength longer.According to the publication, the dopant introduced into the activelayer allows a different emission level to be generated in the energyband of the crystal, resulting in a longer emission wavelength. Thehalf-width of the emission intensity spectrum relative to the emissionwavelength can also be changed depending on the combination of thedopants. If, for example, Zn and Se are the dopants, an emission levelas ZnSe can be established, which is different from the level producedby doping Zn or Se and also different from a simple sum of the levels byZn and Se. The publication, however, does not disclose any specificamount of the dopant(s) introduced into the active layer. Thepublication only discloses a light emitting device formed on a sapphiresubstrate specifically. The publication also fails to note thecrystallinity of each layer formed on the substrate.

[0005] Japanese Patent Laying-Open No. 10-270804 discloses a lightemitting nitride semiconductor device having a light emitting layer (anactive layer) with a multi quantum well structure formed of GaNAs, GaNPor GaNSb well layer/GaN barrier layer. In this publication, a sapphire(α-Al₂O₃) substrate and a SiC substrate are examples of the substrate.The publication does not disclose any doped active layer.

[0006] The light emitting layer formed of GaNAs, GaNP or GaNSb crystalcan provide smaller effective mass of electrons and holes as comparedwith InGaN crystal conventionally used. This suggests that a populationinversion for lasing can be obtained by a small carrier density (andthat a lasing threshold current value can be reduced). If, however, Asis contained in the light emitting layer of nitride semiconductor, thelight emitting layer can readily be separated into a higher nitrogencontent region and a higher As content region. Hereinafter thisphenomenon will be referred to as “composition separation”. The crystalsystem can further be separated into a hexagonal system of the highernitrogen content region and the cubic system of the higher As contentregion. Such a separation into different crystal systems (hereinafterreferred to as “crystal system separation”) can result in a reducedluminous efficacy due to the degraded crystallinity. Such crystal systemseparation can be occurred in a P- or Sb-containing light emittingnitride semiconductor layer as well as in the As-containing layer. Thus,it has been desired that such crystal system separation should beprevented for improved luminous efficacy (or emission intensity).

SUMMARY OF THE INVENTION

[0007] An object of the present invention is to provide a light emittingdevice having a higher luminous efficacy or emission intensity byclarifying a structure capable of enhancing the performance of the lightemitting device using a light emitting layer of nitride semiconductorcontaining at least one of As, P and Sb.

[0008] The present inventors have found that the crystal systemseparation due to As, P or Sb contained in the light emitting nitridesemiconductor layer can be reduced by doping the light emitting layerwith an impurity of at least one element of Mg, Be, Zn, Cd, C, Si, Ge,Sn, O, S, Se and Te, so that the light emitting nitride semiconductordevice can have a good crystallinity and a high luminous efficacy (oremission intensity).

[0009] Thus, the present invention is directed to a light emittingnitride semiconductor device including: one of a substrate made ofnitride semiconductor crystal and a substrate having a nitridesemiconductor crystal film grown on a crystalline material other thanthe nitride semiconductor crystal; an n-type layer and a p-type layereach made of nitride semiconductor formed on the substrate; and a lightemitting layer provided between the n- and p-type layers. The lightemitting layer is formed of a well layer or a combination of well andbarrier layers. Of the layer(s) forming the light emitting layer, atleast the well layer is made of a nitride semiconductor containing anelement X, N and Ga, wherein the element X is at least one selected fromthe group consisting of As, P and Sb. In the nitride semiconductorforming the light emitting layer, the ratio of the number of element Xatoms to the total number of the element X atoms and N atoms, is notmore than 30 atomic percent. Of the layer(s) forming the light emittinglayer, at least the well layer contains as an impurity at least oneelement selected from the group consisting of Mg, Be, Zn, Cd, C, Si, Ge,Sn, O, S, Se and Te.

[0010] In the present invention, the total content of the impurity is1×10¹⁷ to 5×10²⁰/cm³.

[0011] In the present invention, preferably, the nitride semiconductorcrystal or the nitride semiconductor crystal film of the substrate, orthe light emitting nitride semiconductor device has a threadingdislocation density of not more than 3×10⁷/cm² or an etch pit density ofnot more than 7×10⁷/cm².

[0012] In the present invention, typically, the light emitting layer maybe a multi-quantum well layer.

[0013] The present invention is also directed to a light emittingapparatus comprising the light emitting nitride semiconductor device asaforementioned and having an emission wavelength of 380 nm to 650 nm.

[0014] The present invention is also directed to an optical pickupapparatus including a light emission apparatus comprising the lightemitting nitride semiconductor device as aforementioned and having anoscillation wavelength of 380 nm to 420 nm.

[0015] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the drawings:

[0017]FIG. 1 is a schematic cross section of an example of a lightemitting diode device grown on a nitride semiconductor substrate;

[0018]FIG. 2 is a schematic cross section of an example of a quasi GaNsubstrate;

[0019]FIG. 3(a) is a schematic cross section showing the process ofanother example of the quasi GaN substrate, and

[0020]FIG. 3(b) is a schematic cross section showing the completedstructure thereof;

[0021]FIG. 4 is a schematic cross section of another example of thelight emitting diode device according to the present invention;

[0022]FIG. 5 is a top view of the light emitting diode device shown inFIG. 4;

[0023]FIG. 6 is a schematic cross section of an example of a laser diodedevice according to the present invention;

[0024]FIG. 7 schematically shows an optical disc apparatus as oneexample of an information recording apparatus; and

[0025]FIG. 8 represents a relationship between the amount of an impurityadded to the light emitting layer, and the crystal system separation andthe emission intensity of the device;

[0026] In the figures, an n-type GaN substrate is represented by areference numeral 100, a low temperature GaN buffer layer by 101, ann-type GaN layer by 102, a light emitting layer by 103, a p-typeAl_(0.1)Ga_(0.9)N carrier block layer by 104, a p-type GaN contact layerby 105, a transparent electrode by 106, a p electrode by 107, an nelectrode by 108, a dielectric film by 109, a quasi GaN substrate by 200and 200 a, a seed substrate by 201, a low temperature buffer layer by202, an n-type GaN film by 203, a first n-type GaN film by 203 a, asecond n-type GaN film by 203 b, an anti-growth film by 204, an n-typeGaN thick film by 205, the center of the width of the anti-growth filmby 206, the center of the width of the anti-growth film free portion by207, the center of the width of a groove by 208, the center of the widthof the groove free portion, i.e., a plateau by 209, a substrate by 300,an n-type GaN substrate by 400, a low temperature GaN buffer layer by401, an n-type Al_(0.05)Ga_(0.95)N layer by 402, an n-typeIn_(0.07)Ga_(0.93)N anti-crack layer by 403, an n-type Al_(0.1)Ga_(0.9)Nclad layer by 404, an n-type GaN optical guide layer by 405, a lightemitting layer by 406, a p-type Al_(0.2)Ga_(0.8)N carrier block layer by407, a p-type GaN optical guide layer by 408, a p-type Al_(0.1)Ga_(0.9)Nclad layer by 409, a p-type GaN contact layer by 410, an n electrode by411, a p electrode by 412, and a SiO₂ dielectric film by 413.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The device according to the present invention has a substratemade of nitride semiconductor crystal (hereinafter referred to as a“nitride semiconductor substrate”) or a substrate having a nitridesemiconductor crystal film grown on a crystalline material other thanthe nitride semiconductor crystal (hereinafter referred to as a “quasinitride semiconductor substrate”). The nitride semiconductor substrategenerally has a low dislocation density such as 10⁷/cm² or less. Thusthe nitride semiconductor substrate may be used to fabricate a lightemitting nitride semiconductor device having a small threadingdislocation density of at most approximately 3×10⁷/cm², or a small etchpit density of at most approximately 7×10⁷/cm² and hence goodcrystallinity. Such an effect can also be obtained by employing thequasi nitride semiconductor substrate. If the quasi nitridesemiconductor substrate is used, the nitride semiconductor crystal filmgrown on the different crystalline material preferably has a dislocationdensity of at most 10⁷/cm² to reduce the dislocation density of thedevice. The dislocation density may be represented by an etch pitdensity or a threading dislocation density. The etch pit density can beobtained by measuring a pit density on the surface of a test piece suchas a substrate which has been immersed in an etchant of 1:3 ofphosphoric acid and sulfuric acid (at 250° C.) for 10 minutes. Thethreading dislocation density can be measured with a transmissionelectron microscope.

[0028] For the light emitting layer containing As, P, Sb, in particular,a high threading dislocation density results in a reduced luminousefficacy and hence an increased threshold current value. This ispossibly because As, P, or Sb segregates in the vicinity of thethreading dislocation so that the crystallinity can be degraded in thelight emitting layer. The use of the nitride semiconductor substrate orthe quasi nitride semiconductor substrate can prevent such an increaseof a threshold current value and the degradation of the crystallinity inthe light emitting layer. The nitride semiconductor substrate is alsopreferable as it can provide good resonator ends with a small mirrorloss through a cleavage process. The nitride semiconductor substrate hashigh thermal conductivity and thus serves as a good heat sink. Thenitride semiconductor substrate and the nitride semiconductor filmformed thereon can have substantially the same thermal expansioncoefficient, so that the wafer can have little distortion and the yieldof chips through the dividing process can be improved. Thus it isparticularly preferable to use the nitride semiconductor substrate forthe device of the present invention.

[0029] Principle of the Present Invention

[0030] In the conventional GaNAs well layer, crystal system separationcan readily be caused by the As contained in the layer, resulting indegraded crystallinity and reduced luminous efficacy of the lightemitting nitride semiconductor device. This crystal system separationcan be generated in a GaNP or GaNSb well layer as well as in the GaNAswell layer.

[0031] The crystal system separation may be caused by the fact that therate of the adsorption of As, P or Sb to Ga is extremely higher thanthat of N to Ga and the fact that the volatility of N is extremelyhigher than that of As, P or Sb, i.e., N can readily be removed from thecrystal. When a Ga source material and an N source material are suppliedto grow GaN crystal, at the outermost surface (the epitaxial growthsurface) of the growing GaN crystal, a part of the supplied N materialis combined with the Ga material to form the GaN, while most of N,having high volatility, readily re-evaporates. The re-evaporation of Nresults in some Ga failing to form GaN crystal and diffusing in theepitaxial growth surface and then re-evaporating. In such a process, ifa source material of As, P or Sb is supplied, the Ga diffusing in theepitaxial growth surface can readily be adsorbed to As, P or Sb, sincethe rate of the adsorption of As, P or Sb to Ga is extremely higher thanthat of N to Ga. Thus the Ga—As, Ga—P or Ga—Sb bond is formed morepreferentially than the Ga—N bond. In addition, Ga has a long surfacemigration length, which can give a high probability of the collision ofthe Ga—As, Ga—P or Ga—Sb bonds. At the collision, the bonds are fixed tofacilitate crystallization. This is a segregation effect due to theabove-mentioned bond. This segregation effect results in a compositionseparation into a portion with a high content of the bond and a portionwith a low content of the bond. When the composition separation hasadvanced, the portion with the high content of the bond finally forms acubic crystal system and the portion with the low content of the bondfinally forms a hexagonal crystal system. This is referred to as crystalsystem separation.

[0032] In the present invention, the crystal system separation isprevented by doping the light emitting nitride semiconductor layer withat least one of Mg, Be, Zn, Cd, C, Si, Ge, Sn, O, S, Se and Te. Theimpurity is distributed uniformly across the entire epitaxial growthfilm to form a nucleus for crystal growth. This nucleus traps the Ga—As,Ga—P or Ga—Sb bond. More specifically, the introduction of the impurityto form the nucleus substantially reduces the surface migration lengthof Ga. Thus, doping the entire surface of the epitaxial growth filmuniformly with the impurity can reduce the collision of the Ga—As, Ga—Por Ga—Sb bonds, so that localized significant crystallization can beprevented (i.e., the segregation effect can be reduced). Thus, thecrystal system separation can be prevented to improve the crystallinityof the light emitting layer. The adsorption of the Ga atom to the GroupV is described in the above by way of illustration. The above-describedmechanism is also applicable to other Group III atoms than Ga.

[0033] Relationship Between Doping and Crystal Defects in Light EmittingLayer in the Present Invention

[0034] The relationship between the doping according to the presentinvention and the crystal defects (mainly the threading dislocation)will be described referring to FIG. 8. FIG. 8 shows the degree of thecrystal system separation generated in a Si-doped GaN_(0.92)P_(0.08)well layer having an emission wavelength of 520 nm, and the emissionintensity. Herein the “degree of crystal system separation” refers tothe ratio by volume (in percentage) of the portion with the crystalsystem separation to the crystal system separation free portion (that isformed with the average composition ratio) in a unit volume of the welllayer. In FIG. 8, the horizontal axis represents the amount of Si dopantintroduced, and the left vertical axis represents the degree (%) of thecrystal system separation and the right vertical axis represents theemission intensity in an arbitrary unit. In FIG. 8, the emissionintensity is standardized to have a level of one when the well layer isnot doped with the impurity. In FIG. 8, the circle represents thecharacteristics of the light emitting device with the well layer grownon a GaN substrate (an example of the nitride semiconductor substrate),and the square represents the characteristics of the light emittingdevice with the well layer grown on a sapphire substrate. As shown inthe figure, the light emitting device grown on the GaN substrate has athreading dislocation density of approximately 1×10⁷/cm² and an etch pitdensity of not more than approximately 5×10⁷/cm², and the light emittingdevice grown on the sapphire substrate has a threading dislocationdensity of approximately 1 to 10×10⁹/cm² and an etch pit density of notless than approximately 4×10⁸/cm². The etch pit density can be obtainedby measuring a pit density on the surface of the epitaxial wafer (thelight emitting device) which has been immersed in an etchant of 1:3 ofphosphoric acid and sulfuric acid (at 250° C.) for 10 minutes. The etchpit density is obtained as a result of measuring the surface of theepitaxial wafer of the light emitting nitride semiconductor device,which is not a result of directly measuring the defects in the lightemitting layer. However, if the etch pit density is high, the lightemitting layer has a high defect density, and therefore the measuredetch pit density will indicate whether the active layer has a largenumber of defects.

[0035]FIG. 8 demonstrates that the crystal system separation can bereduced by the doping according to the present invention moreeffectively in the light emitting device grown on the GaN substrate thanin that grown on the sapphire substrate. The figure also shows that thedevice on the GaN substrate has greater emission intensity.

[0036] As well as the GaN substrate as described above, a substratehaving a structure with a GaN crystal film grown on a crystallinematerial other than GaN crystal (hereinafter referred to as a “quasi GaNsubstrate”) is also preferable. The quasi GaN substrate may be producedas described in detail below. In the nitride semiconductor films grownon the quasi GaN substrate, the smallest threading dislocation densityis not more than approximately 3×10⁷/cm² and the smallest etch pitdensity is not more than approximately 7×10⁷/cm². These values are closeto those of the nitride semiconductor film grown on the GaN substrate.However, the quasi GaN substrate has different portions with low andhigh threading dislocation densities in a mixed manner and therefore itcan provide a lower yield of the light emitting device than the GaNsubstrate (the nitride semiconductor substrate). In the light emittingdevice grown on the quasi GaN substrate, the relationship between theamount of Si dopant introduced and the degree of the crystal systemseparation and the device's emission intensity is almost the same asthat in the GaN substrate as shown in FIG. 8. If the quasi GaN substrateis used, to obtain such a result as shown in FIG. 8, the light emittingdevice is desirably grown on portions with less crystal defects (or lessthreading dislocations).

[0037] Thus it has been found that the emission intensity of the lightemitting device with less crystal defects (mainly threadingdislocations) is greater than that with more crystal defects, even ifboth devices contain the impurity in the same concentration. Thus thecrystal defects may also trap the Ga—As, Ga—P or Ga—Sb bond as well asthe nucleus formation by the impurity. However, the function of thecrystal defects to trap the bond is significantly greater than that ofthe nucleus formation by the impurity and therefore the crystal defectsmay promote the segregation rather than reduce the crystal systemseparation. The crystal defects are not uniform and the threadingdislocation, which is a main defect of the crystal defects, is in theform of a pipe having a diameter on the order of several nm to severaltens nm. Such crystal defects may cause a significant segregationeffect. In contrast, the nucleus formation by the impurity should bedistributed uniformly across the entire epitaxial growth film.

[0038] As can be seen from the foregoing, in order to improve theemission intensity, it is desirable that the light emitting layer isdoped with the impurity and the GaN substrate (the nitride semiconductorsubstrate) or the quasi GaN substrate is used for the light emittingdevice. It has also been found that a threading dislocation of not morethan approximately 3×10⁷/cm² or an etch pit density of not more thanapproximately 7×10⁷/cm² significantly improve the effect of introducingthe impurity in the present invention.

[0039] The obtained characteristics of the device having a lightemitting nitride semiconductor layer containing any one of As, P and Sbare similar to those as shown in FIG. 8. A similar effect can also beobtained when Si dopant is replaced by Mg, Be, Zn, Cd, C, Ge, Sn, O, S,Se or Te dopant.

[0040] Impurity in Well Layer According to the Present Invention and ItsAmount

[0041] A description will now be provided of the impurity and its amountto be introduced to produce the effect of the present invention.

[0042] Initially, experiments are carried out to reveal the amount ofAs, P or Sb for causing the above-described crystal system separation.As a result, when the GaN crystal is doped with As, P or Sb of1×10¹⁸/cm² or more, the crystal system separation starts (with a crystalsystem separation degree of approximately 2 to 3%), and the degreeattains to approximately 12 to 13% when the number of the element atomsintroduced amounts to approximately 10 atomic percent of the totalnumber of the Group V element atoms in the nitride semiconductor.

[0043] Referring again to FIG. 8, the relationship between the amount ofthe impurity introduced, and the crystal system separation and theemission intensity will be described. In the figure, as indicated byhollow circles, the degree of the crystal system separation started todecrease (to 10% or less) at a dopant amount of approximately1×10¹⁷/cm³, was approximately 6% or less at approximately 5×10¹⁷/cm³,started to gradually increase at approximately 2×10¹⁹/cm³, abruptlyincreased at more than 1×10²⁰/cm³, and was 10% or more at more than5×10²⁰/cm³. On the other hand, as indicated by solid circles, similarly,the emission intensity started to increase at a dopant amount ofapproximately 1×10¹⁷/cm³, abruptly increased at approximately5×10¹⁷/cm³, had a peak around 5×10¹⁸/cm³, started to gradually decreasearound 2×10¹⁹/cm³, abruptly decrease at more than 1×10²⁰/cm³, and was nolonger superior at more than 5×10²⁰/cm³. This shows that there is acorrelation between the crystal system separation and the emissionintensity.

[0044] In FIG. 8, as indicated by the circles, the crystal systemseparation was not prevented at a dopant amount of less than 1×10¹⁷/cm³.This may be because at such a dopant amount, the residual crystaldefects can trap As, P or Sb more strongly than the impurity. On theother hand, the crystal system separation started to gradually increaseat a dopant amount of more than 2×10^(19/cm) ³. This may be because thecrystallinity of the light emitting layer is degraded by the dopingitself.

[0045] As can be seen from the detail of FIG. 8, also in the lightemitting device grown on a sapphire substrate, which is indicated bysquares, the degree of the crystal system separation gradually decreasesas the impurity is introduced, and the emission intensity accordinglygradually increases. Such an effect of reducing the crystal systemseparation is, however, different from that of the device on the GaNsubstrate. The light emitting device grown on the sapphire substrate hasa threading dislocation density higher than that on the GaN substrate,so that it cannot efficiently exhibit the effect of the introducedimpurity. If the impurity concentration is not higher than that in thedevice on the GaN substrate, the crystal system separation cannot bereduced effectively. As for the squares, it was expected that thecrystal system separation would further be reduced by doping at not lessthan approximately 2×10¹⁹/cm³. In fact, however, the degree of thecrystal system separation increases as shown in FIG. 8. The excessivelyintroduced impurity seems to degrade the crystallinity so significantlythat the crystal system separation increases.

[0046] Thus, for a high emission intensity or luminous efficacy in thelight emitting device using a GaN substrate or a quasi GaN substratewith a low threading dislocation, the degree of the crystal systemseparation is preferably not more than 10%, and more preferably not morethan approximately 6%. Such a degree of the crystal system separationcan be obtained by introducing the impurity of 1×10¹⁷/cm³ to 5×10²⁰/cm³and preferably 5×10¹⁷/cm³ to 1×10²⁰/cm³.

[0047] A similar effect was also obtained when the Si dopant wasreplaced by Mg, Be, Zn, Cd, C, Ge, Sn, O, S, Se or Te dopant. When aplurality kinds of the above dopants were introduced, a similar effectwas also obtained. If a plurality kinds of dopants are introduced, thetotal amount thereof is desirably 1×10¹⁷/cm³ to 5×10²⁰/cm³.

[0048] A barrier layer may be doped or may not be doped, since thebarrier layer does not directly contribute to the light emitting throughthe recombination of the injected carriers. If the barrier layercontains at least one of As, P and Sb, however, preferably it is dopedwith the impurity as well as the well layer. By doing so, thecrystallinity of the barrier layer can also be improved, as describedabove.

[0049] Impurity in the Present Invention

[0050] The impurity suitable for the present invention is Mg, Be, Zn,Cd, C, Si, Ge, Sn, O, S, Se or Te. These impurity elements are generallyclassified into Group II elements Be, Mg, Zn, Cd, Group IV elements C,Si, Ge, Sn, and Group VI elements O, S, Se, Te.

[0051] The ionic bonding of Group IV elements is weaker than that ofGroup II or VI elements (to be close to covalent bond) and it mainly,simply inhibits a Ga—As, Ga—P or Ga—Sb bond from diffusing in thesurface (or substantially reduces the surface migration length). A partof the ionic bonding of Group IV elements is less substituted by theimpurity as compared with the case of the Group II or VI elements. Inthe case of the Group IV elements, therefore, the sift level of theemission wavelength is not so significant, and the collision of thebonding and the localized large crystallization can be prevented (i.e.,the effect of reducing the crystal system separation can be obtained)only by controlling the amount of the impurity.

[0052] Of the Group IV elements, Si is particularly preferable and C andGe are the next preferable elements in order of decreasing thesingle-bond energy to N. If the element has a higher single-bond energyto N, such an element hardly combines with N. In the present invention,the crystal system separation is prevented by reducing the segregationof As, P or Sb. In the present invention, the impurity that adsorbs As,P, Sb rather than N is preferably used.

[0053] The Group II elements forms positive ions and thus not onlyinhibit the Ga—As, Ga—P or Ga—Sb bond from diffusing in the surface butattract and adsorb the bond. Therefore, the amount of the Group IIelement introduced for efficiently preventing the crystal systemseparation can be smaller than that of the Group IV element. In the caseof the Group II elements, the degree of the crystal separation startedto decrease at a dopant amount of approximately 5×10¹⁶/cm³, was minimumaround 1×10¹⁸/cm³, and started to increase around 1×10²⁰/cm³ or more. Ifthe amount of the impurity introduced is small, the degradation of thecrystallinity by the doping itself can be reduced.

[0054] The Group III elements (such as Al, Ga or In) that can combinewith As, P or Sb can be replaced with the Group II impurity. When suchreplacement is occurred, As, P or Sb remains in the crystal and theother Group III elements can re-evaporate, so that the emissionwavelength of the light emitting device can be sifted to a somewhatlonger wavelength. In the case that the Group II elements are used fordoping, therefore, it can be more difficult to obtain the targetedemission wavelength through the process as compared with the case thatthe Group IV elements are used for doping. If, however, the lightemitting layer should contain so much As, P or Sb that the mixed crystalcontent can be high (i.e., a long-wavelength light emitting device ofapproximately 450 nm or more should be produced), the control of theamount of the Group II element introduced can be easier than the controlof the amount of the supplied As, P or Sb source material. This isbecause in such a case, a proportional relationship cannot beestablished between the amount of the supplied As, P or Sb material andthe composition ratio of the light emitting layer.

[0055] The Group VI elements forms negative ions and therefore they notonly inhibit the Ga—As, Ga—P or Ga—Sb bond from diffusing in the surfacebut attract and adsorb the bond effectively. Therefore, the amount ofthe Group VI element introduced for efficiently preventing the crystalsystem separation can be smaller than that of the Group IV element. Inthe case of the Group VI elements, for example, the degree of thecrystal separation started to decrease at a dopant amount ofapproximately 5×10¹⁶/cm³, was minimum around 1×10¹⁸/cm³, and started toincrease around 1×10²⁰/cm³ or more. If the amount of the impurityintroduced is small, the degradation of the crystallinity by the dopingitself can be reduced.

[0056] The Group V elements (such as P, As or Sb) that can combine withthe Group III element such as Ga can be replaced with the Group VIimpurity. When such replacement is occurred, As, P or Sb re-evaporatesfrom the crystal, so that the emission wavelength of the light emittingdevice can be sifted to a somewhat shorter wavelength. In the case thatthe Group VI elements are used for doping, therefore, it can be moredifficult to obtain the targeted emission wavelength through the processas compared with the case that the Group IV elements are used fordoping. If, however, the content of As, P or Sb in the light emittinglayer should be so low that the mixed crystal content can be low (i.e.,a short-wavelength light emitting device of approximately 450 nm or lessshould be produced), the control of the amount of the Group VI elementintroduced can be easier than the control of the amount of the suppliedAs, P or Sb source material. This is because As, P or Sb is lessvolatile that N and can be easily incorporated into the light emittinglayer.

[0057] Preferable impurities for the As or P containing light emittinglayer will be described below.

[0058] Impurities for As-containing Light Emitting Layer

[0059] If the light emitting layer contains As, Ge or Si is mostpreferable dopant, which is the Group IV element. Because the covalentbond radii of Ge and Si (approximately 0.122 nm and approximately 0.117nm, respectively) are close to that of As (approximately 0.121 nm), Geand Si seems to be able to trap As readily and appropriately.

[0060] Second preferable is Mg or Zn, which is the Group II element. Theionic radii of Mg and Zn are approximately 0.065 nm and 0.074 nm,respectively, which are close to that of Ga (0.062 nm), which is a GroupIII element and a main component of the light emitting layer. Thus if Gais replaced with Mg or Zn as described above, defects or distortion canpreferably be reduced in the light emitting layer.

[0061] Third preferable is C, which is the Group IV element. Thecovalent bonding radius of C is approximately 0.077 nm, which issignificantly close to that of N (0.070 nm), which a Group V element anda main component of the light emitting. Therefore, if the light emittinglayer is made of C dopant containing nitride semiconductor crystal,distortion or defects of the crystal due to the doping can be reduceddue to its covalent bonding radius significantly close to that of N, amain component of the light emitting layer.

[0062] Impurities for P-containing Light Emitting Layer If the lightemitting layer contains P, Si is most preferable dopant, which is theGroup IV element. Because the covalent bond radius of Si (approximately0.117 nm) is close to that of P (approximately 0.110 nm), Si seems to beable to trap P readily and appropriately.

[0063] Second preferable is Mg or Zn, which is the Group II element. Theionic radii of Mg and Zn are approximately 0.065 nm and 0.074 nm,respectively, which are close to that of Ga (0.062 nm), which is a GroupIII element and a main component of the light emitting layer. Thus if Gais replaced with Mg or Zn as described above, defects or distortion canpreferably be reduced in the light emitting layer.

[0064] Third preferable is C, which is the Group IV element. Thecovalent bonding radius of C is approximately 0.077 nm, which issignificantly close to that of N (0.070 nm), which a Group V element anda main component of the light emitting. Therefore, if the light emittinglayer is made of C dopant containing nitride semiconductor crystal,distortion or defects of the crystal due to the doping can be reduceddue to its covalent bonding radius significantly close to that of N, amain component of the light emitting layer.

[0065] Process of Introducing the Impurity

[0066] In the process of introducing the impurity, the impurity may beintroduced before the light emitting layer (a well layer or a barrierlayer) is formed, or the impurity may be introduced in the growthprocess of the light emitting layer (a well layer or a barrier layer).

[0067] In the former manner, i.e., when the impurity is used before thegrowth process of the light emitting layer, the impurity can form acrystal nucleus before the light emitting layer is grown. Thus, thecrystal system separation can be prevented efficiently from the initialstage of forming the light emitting layer. This can prevent the lightemitting layer from being affected by the underlying layer and fromhaving the crystal system separation. The former manner is effective inthe case that the As, P or Sb containing light emitting layer has arelatively small thickness.

[0068] In the latter manner, i.e., when the impurity is introducedduring the growth of the light emitting layer, the impurity forms acrystal nucleus in the process of the light emitting layer growth, sothat the crystal system separation slightly remains in the lightemitting layer. In this case, however, the introduction of the impuritysynchronized with the growth of the light emitting layer can providerespective layers with the crystal system separations prevented in thesame manner in the crystal growth direction. That is, in the lattermanner, the local generation of an intensive crystal system separationcan be prevented in the light emitting layer, so that the crystallinitycan be uniform over the layer. Thus the latter manner is effective ingrowing a thick light emitting layer.

[0069] Preferably, the impurity is dispersed in the light emitting layeruniformly. This can effectively reduce the substantial surface migrationlength of the Ga—As, Ga—P or Ga—Sb bond and thus prevent the bond frombeing locally fixed into segregation. The impurity is preferablyintroduced by the method using a gaseous impurity material, such asMOCVD.

[0070] Light Emitting Layer for the Present Invention

[0071] In the present invention, the light emitting layer may be formedsimply of a well layer, or it may have a structure in which well andbarrier layers are stacked alternately. In the present invention, of thelayers constituting the light emitting layer, at least the well layer ismade of a nitride semiconductor containing an element X that is at leastone selected from the group consisting of As, P and Sb. If the lightemitting layer is composed of the combination of well and barrierlayers, only the well layer(s) may be formed of such a nitridesemiconductor or the well and barrier layers may be formed of such anitride semiconductor. Such a nitride semiconductor further contains Gaand N. In such a nitride semiconductor, element X has an atomic fractionsmaller than N. Additionally, in such a nitride semiconductor, the ratioof the number N₁ of element X to the total number of number N₁ and thenumber N₂ of element N, is not more than 30 atomic percent, preferablynot more than 20 atomic percent. The layer formed of such a nitridesemiconductor (the well layer or the well and barrier layers) preferablyhas an element X concentration of not less than 1×10¹⁸/cm³. If{N₁/(N₁+N₂)}×100(%) exceeds 30%, the introduction of the impurity cannotachieve sufficient prevention of the crystal system separation and thecrystallinity of the light emitting layer is degraded. If{N₁/(N₁+N₂)}×100(%) is not more than 20%, the impurity introduced canprevent the crystal system separation more effectively. If the element Xconcentration is not less than 1×10¹⁸/cm³, the introduction of theimpurity can significantly restrain the crystal system separation. Sucha composition of the nitride semiconductor can restrain the crystalsystem separation as described above to make the crystallinity of thewell layer good, resulting in a high emission intensity or a low lasingthreshold current density. A similar mechanism can be applied to thebarrier layer, although the barrier layer is not required to contain As,P or Sb and it is only required to have a greater band gap energy thanthe well layer.

[0072] In the present invention, the nitride semiconductor forming atleast the well layer can typically be represented by the formulaIn_(x)Al_(y)Ga_(1-x-y)N_(t)As_(u)P_(v)Sb_(z), wherein 0≦x<1, 0≦y<1, and0<u+v+z<t. In the formula, t+u+v+z may be one. At least one of u, v andz is not zero. (u+v+z)/(u+v+z+t) is not more than 0.3 and preferably notmore than 0.2.

[0073] Thickness of Light Emitting Layer

[0074] In the present invention, the well layer is preferably 0.4 nm to20 nm in thickness. If the well layer has a thickness smaller than 0.4nm, a carrier confinement level by the quantum well effect can be toohigh such that the luminous efficacy can be reduced. If the well layerhas a thickness greater than 20 nm, crystallinity is degraded, dependingon the As, P, Sb content in the well layer.

[0075] In the present invention, the barrier layer is preferably 1 nm to20 nm in thickness. If the barrier layer has a thickness smaller than 1nm, carriers might be confined insufficiently. If the barrier layer hasa thickness greater than 20 nm, it would be difficult to form a subbandstructure for the multi quantum well layer.

[0076] Structure of Light Emitting layer

[0077] In the present invention, the light emitting layer is typicallycomposed of the combination of the well and barrier layers as shown inTable 1. Alternatively, the light emitting layer may have a compositioncontaining the Group III element(s) presented in Table 1 and N, and twoor more additional elements selected from the group consisting of As, Pand Sb. The total content of the additional elements relative to all ofthe Group V elements forming the light emitting layer, is not more than30 atomic percent, preferably not more than 20 atomic percent. In Table1, for the light emitting layer in the present invention, the triangleindicates applicable combinations, the circle indicates preferablecombinations, and the double circle indicates most preferablecombinations. If the light emitting layer has a mono-quantum wellstructure formed simply of a well layer, the well layers in Table 1 thatcontain Sb can be marked by the triangle and the remainder can be markedby the double circles. TABLE 1 Barrier layer GaN GaNAs GaNP GaNSb InGaNInGaNAs InGaNP InGaNSb AlGaN GaNAs ⊚ ◯ ◯ Δ ⊚ ◯ ◯ Δ ◯ GaNP ⊚ ◯ ◯ Δ ⊚ ◯ ◯Δ ◯ GaNSb Δ Δ Δ Δ Δ Δ Δ Δ Δ InGaNAs ⊚ ◯ ◯ Δ ⊚ ◯ ◯ Δ ◯ InGaNP ⊚ ◯ ◯ Δ ⊚ ◯◯ Δ ◯ InGaNSb Δ Δ Δ Δ Δ Δ Δ Δ Δ AlGaNAs ⊚ ◯ ◯ Δ ⊚ Δ Δ Δ ⊚ AlGaNP ⊚ ◯ ◯ Δ⊚ Δ Δ Δ ⊚ AlGaNSb Δ Δ Δ Δ Δ Δ Δ Δ Δ InAlGaNAs ⊚ ◯ ◯ Δ ⊚ Δ Δ Δ ⊚ InAlGaNP⊚ ◯ ◯ Δ ⊚ Δ Δ Δ ⊚ InAlGaNSb Δ Δ Δ Δ Δ Δ Δ Δ Δ Barrier layer AlGaNAsAlGaNP AlGaNSb InAlGaN InAlGaNAs InAlGaNP InAlGaNSb GaNAs Δ Δ Δ ◯ Δ Δ ΔGaNP Δ Δ Δ ◯ Δ Δ Δ GaNSb Δ Δ Δ Δ Δ Δ Δ InGaNAs Δ Δ Δ ◯ Δ Δ Δ InGaNP Δ ΔΔ ◯ Δ Δ Δ InGaNSb Δ Δ Δ Δ Δ Δ Δ AlGaNAs ◯ ◯ Δ ◯ Δ Δ Δ AlGaNP ◯ ◯ Δ ◯ Δ ΔΔ AlGaNSb Δ Δ Δ Δ Δ Δ Δ InAlGaNAs ◯ ◯ Δ ⊚ ◯ ◯ Δ InAlGaNP ◯ ◯ Δ ⊚ ◯ ◯ ΔInAlGaNSb Δ Δ Δ Δ Δ Δ Δ

[0078] As described above, the addition of the impurity to the As, P orSb containing light emitting layer can reduce the crystal systemseparation and improve the sharpness of the interface between the welland barrier layers. This facilitates fabricating the combinations of thewell and barrier layers presented in Table 1 (multi-quantum wellstructures). In contrast, a conventional, impurity-free light emittinglayer contains portions having different crystal systems in a mixedmanner, so that the sharpness of the interface between its well andbarrier layers gets significantly worse as the number of the stackedlight emitting layers increases. Such degradation in the sharpness ofthe interface makes it difficult to fabricate a multilayered structure(a multi-quantum well structure) itself and the light emitting layeralso provides an unevenness of color and reduced emission intensity.According to the present invention, such disadvantage of the prior artcan be overcome by the addition of the impurity to the light emittingnitride semiconductor containing at least one of As, P and Sb and themulti-quantum well structure can easily be formed. Preferably, themulti-quantum well structure provides an emission intensity greater thana mono-quantum well structure and provides a laser diode with a smallerthreshold current density. More specific compositions of the well andbarrier layers forming the light emitting layer will be described below.

[0079] GaNX Well Layer (X is As, P, Sb or Any Combination Thereof)

[0080] If a well layer is formed of GaNX crystal, it does not contain Inand is thus free of the In segregation-induced composition separation.The In composition separation herein means that a single layer isseparated into a region with a high In content and a region with a lowIn content (and the regions are mixed in the layer). The well layer freeof the In-induced composition separation does not have a non-lightemitting region caused by a high In content and it can preferably befree of a factor increasing the threshold current value of the device.

[0081] Of GaNX crystals, the 3-element mixed crystal of GaNAs, GaNP orGaNSb has a composition easier to control than the 4-element mixedcrystal of GaNAsP and the 5-element mixed crystal of GaNAsPSb. Thus thetargeted wavelength can be obtained in a good reproducibility. Of P, Asand Sb, P has a atomic radius (a Van der Waals radius or covalent bondradius) closest to that of N and therefore it can displace a portion ofthe N atoms in the mixed crystal more easily than As and Sb. Thus GaNwith P added thereto, or GaNP, can have good crystallinity. Thissuggests that an increased P content in GaNP may not so severely degradethe crystallinity of the mixed crystal. When the light emitting deviceuses a GaNP well layer, the GaNP crystal can cover a wide emissionwavelength range from ultra violet light emission to red light emission.

[0082] Of P, As, and Sb, Sb has the largest atomic radius (or Van derWaals radius or covalent bond radius) relative to that of N, and ascompared to As and Sb, it has a weaker tendency to displace a portion ofthe N atoms in the mixed crystal. However, the Sb atomic radius greaterthan that of As and P can prevent the removal of highly volatile N atomsfrom the mixed crystal and thus make the crystallinity of GaNSb good.

[0083] The atomic radius of As is intermediate between those of P and Sband therefore GaNAs can preferably have both characteristics of GaNP andGaNSb.

[0084] The emission wavelength of the light emitting layer employing theGaNX well layer can be modified by the modulation of the As, P or Sbcontent ratio in the well layer. For example, to obtain a emissionwavelength around UV 380 nm, in GaN_(1-x)As_(x) x should be 0.001, inGaN_(1-y)P_(y) y should be 0.01, and in GaN_(1-z)Sb_(z) z should be0.02. To obtain an emission wavelength around 410 nm of blue-violetcolor, in GaN_(1-x)As_(x) x should be 0.02, in GaN_(1-y)P_(y) y shouldbe 0.03, and in GaN_(1-z)Sb_(z) z should be 0.01. To obtain a wavelengtharound 470 nm of blue color, in GaN_(1-x)As_(x) x should be 0.03, inGaN_(1-y)P_(y) y should be 0.06, and in GaN_(1-z)Sb_(z) z should be0.02. To obtain a wavelength around 520 nm of green color, inGaN_(1-x)As_(x) x should be 0.05, in GaN_(1-y)P_(y) y should be 0.08,and in GaN_(1-z)Sb_(z) z should be 0.03. To obtain a wavelength around650 nm of red color, in GaN_(1-x)As_(x) x should be 0.07, inGaN_(1-y)P_(y) y should be 0.12, and in GaN_(1-z)Sb_(z) z should be0.04. The above composition ratios or near ratios can complete thetargeted emission wavelength.

[0085] When Al is added to the GaNX well layer, the As, P or Sb contentshould be higher than that for the aforementioned emission wavelengths,because the Al added increases the band gap energy. The addition of Alto the GaNX well layer is preferable, however, because the crystallinityof the well layer can be improved. The N element in the GaNX well layeris significantly more volatile than As, P and Sb, and N can readily beremoved from the crystal, so that the crystallinity of the well layercan be degraded. When Al is added to the GaNX well layer, Al that ishighly reactive can strongly combine with N, so that the removal of Nfrom the well layer can be prevented and the degradation incrystallinity can be reduced.

[0086] The GaNX well layer is preferably combined with a barrier layerof GaN, GaNAs, GaNP, InGaN, InGaNAs, InGaNP, AlGaN or InAlGaN.Particularly, in GaN, InGaN, AlGaN, which are a 2-element mixed crystalor a 3-element mixed crystal composed of two types of Group III elementsand one type of a Group V element, the composition can readily becontrolled and therefore desired compounds can be formed in a goodreproducibility. In particular, InGaN is preferable as its crystallinitycan be better than that of GaN or AlGaN when it is produced at thetemperature range for growing the GaNX well layer, such as 600° C. to800° C. When the barrier layer is made of GaN, the crystallinity ofwhich can be better than that of AlGaN, the interface between the welland barrier layers can be so flat that the luminous efficacy can beimproved.

[0087] InGaNX Well Layer

[0088] When the well layer is formed of InGaNX crystal, it can have thecomposition separation due to the effect of the In segregation. Like In,however, As, P or Sb can reduce the band gap energy of the well layer,and therefore the In content in the InGaNX well layer can be lower thatthat in the conventional InGaN well layer to give the targeted emissionwavelength. When at least one of As, P and Sb is added to theIn-containing well layer, the content of In can be low (so that thecomposition separation can be reduced) while the well layer can havemoderate In segregation. The moderate In segregation can provide alocalized level for the trap of the carriers of the electrons and holes,so that the luminous efficacy can be improved and the threshold currentvalue can be lowered.

[0089] Of InGaNX crystals, the 4-element mixed crystal of InGaNAs,InGaNP or InGaNSb can have a composition easier to control than the5-element mixed crystal of InGaNAsP and the 6-element mixed crystal ofInGaNAsPSb, so that the targeted emission wavelength can be provided ina good reproducibility.

[0090] Of P, As, and Sb, P has an atomic radius (a Van der Waals radiusor covalent bond radius) closest to that of N, and as compared to As andSb, it has a stronger tendency to displace a portion of the N atoms inthe mixed crystal. Thus InGaN with P added thereto, or InGaNP, can havea good crystallinity. This suggests that an increased P content inInGaNP may not so severely degrade the crystallinity of the mixedcrystal. When the light emitting device uses a InGaNP well layer, theInGaNP crystal can cover a wide emission wavelength range from ultraviolet light emission to red light emission.

[0091] Of P, As, and Sb, Sb has the largest atomic radius (or Van derWaals radius or covalent bond radius) relative to that of N, and ascompared with As or Sb, it has a weaker tendency to displace a portionof the N atoms in the mixed crystal. However, the Sb atomic radiusgreater than that of As and P can prevent the removal of highly volatileN atoms from the mixed crystal and thus make the crystallinity ofInGaNSb good.

[0092] The atomic radius of As is intermediate between those of P and Sband therefore InGaNAs can preferably have both characteristics of InGaNPand InGaNSb.

[0093] The emission wavelength of the light emitting layer employing theInGaNX well layer can be modified by the modulation of the As, P or Sbcontent in the well layer. For example, Table 2 presents a relationshipbetween the compositions of InGaNAs and InGaNP, and the emissionwavelength. In preparing the well layer, the compositions shown in Table2 or near compositions can complete the targeted emission wavelength.TABLE 2 In_(y)Ga_(1−y)N_(1−x)As_(x) In(y = 0.01) In(y = 0.02) In(y =0.05) In(y = 0.1) In(y = 0.2) In(y = 0.35) Emission wavelength 380 nm0.005 0.004 0.001 400 nm 0.012 0.011 0.008 0.003 410 nm 0.016 0.0150.011 0.006 470 nm 0.034 0.033 0.029 0.024 0.014 0.001 520 nm 0.0460.045 0.041 0.036 0.025 0.012 650 nm 0.07 0.069 0.065 0.059 0.048 0.034In_(y)Ga_(1−y)N_(1−x)P_(x) In(y = 0.01) In(y = 0.02) In(y = 0.05) In(y =0.1) In(y = 0.2) In(y = 0.35) Emission wavelength 380 nm 0.008 0.0060.001 400 nm 0.02 0.018 0.013 0.004 410 nm 0.025 0.023 0.018 0.01 470 nm0.055 0.053 0.047 0.038 0.022 0.001 520 nm 0.075 0.073 0.067 0.058 0.0410.019 650 nm 0.116 0.114 0.107 0.097 0.079 0.055

[0094] When Al is added to the InGaNX well layer, the In content and theAs, P or Sb content should be higher than those for the emissionwavelengths as shown in Table 2, because the Al added increases the bandgap energy. The addition of Al to the InGaNX well layer is preferable,however, because the crystallinity of the well layer can be improved.The N element in the InGaNX well layer is significantly more volatilethan As, P and Sb, and N can readily be removed from the crystal, sothat the crystallinity of the well layer can be degraded. When Al isadded to the InGaNX well layer, Al that is highly reactive can stronglycombine with N, so that the removal of N from the well layer can beinhibited.

[0095] The InGaNX well layer is preferably combined with a barrier layerof GaN, GaNAs, GaNP, InGaN, InGaNAs, InGaNP, AlGaN or InAlGaN.Particularly, in GaN, InGaN, AlGaN, which are a 2-element mixed crystalor a 3-element mixed crystal composed of two types of Group III elementsand one type of a Group V element, the composition can readily becontrolled and therefore desired compounds can be formed in a goodreproducibility. In particular, InGaN is preferable as its crystallinitycan be better than that of GaN or AlGaN when it is produced at thetemperature range for growing the InGaNX well layer, such as 600° C. to800° C. When the barrier layer is made of GaN, the crystallinity ofwhich can be better than that of AlGaN, the interface between the welland barrier layers can be so flat that the luminous efficacy can beimproved.

EXAMPLE 1

[0096] A light emitting device having the structure as shown in FIG. 1was fabricated. Referring to FIG. 1, the light emitting nitridesemiconductor diode device is composed of an n-type GaN substrate 100having the C (0001) plane, a low temperature GaN buffer layer 101 (of100 nm in thickness), an n-type GaN layer 102 (having a thickness of 3μm and a Si impurity concentration of 1×10¹⁸/cm³), a light emittinglayer 103, a p-type Al_(0.1)G_(0.9)N carrier block layer 104 (having athickness of 20 nm and a Mg impurity concentration of 6×10¹⁹/cm³), ap-type GaN contact layer 105 (having a thickness of 0.1 μm and a Mgimpurity concentration of 1×10²⁰/cm³), a transparent electrode 106, a pelectrode 107, and an n electrode 108. The device was fabricated by thefollowing process.

[0097] First, in a metal-organic chemical vapor deposition (MOCVD)apparatus, n-type GaN substrate 100 was placed, and NH₃ (ammonia) thatis a Group V source material, and TMGa (trimethylgallium) or TEGa(tryethylgallium) that is a Group III source material, were used to growlow temperature GaN buffer layer 101 at 550° C. to have a thickness of100 nm. Then at 1050° C. SiH₄ (silane) was added to the source materialsand n-type GaN layer 102 (having a Si impurity concentration of1×10¹⁸/cm³) of 3 μm in thickness was formed. Then the substratetemperature was decreased to 800° C., and while SiH₄ was introduced as aSi impurity source, PH₃ or TBP (t-butylphosphine) was introduced as a Psource material to grow GaN_(0.92)P_(0.08) light emitting layer 103 of 4nm thick. This light emitting layer has a single quantum well structure.

[0098] If As is added to the light emitting layer, AsH₃ or TBAs(t-butylarsine) may be used. If Sb is added to the light emitting layer,TMSb (trimethylantimony) or TESb (triethylantimony) may be used. Informing the light emitting layer, dimethylhydrazine may be used in placeof NH₃ as the N source material.

[0099] Then the substrate temperature was increased again to 1050° C.and TMAl (trimethylaluminum) or TEAl (triethylaluminum) that is a GroupIII source material was used to grow p-type Al_(0.1)Ga_(0.9)N carrierblock layer 104 of 20 nm thick and subsequently grow p-type GaN contactlayer 105 of 0.1 μm thick. As the p-type impurity, Mg was added in aconcentration of 5×10¹⁹/cm³ to 2×10²⁰/cm³. The source of Mg was EtCP₂Mg(bisethylcyclopentadienylmagnesium). Preferably, p-type GaN contactlayer 105 has a p-type impurity concentration increasing as itapproaches the location at which transparent electrode 106 is formed.Such an impurity distribution can prevent the crystal defects from beingincreased by the impurity introduction, and can reduce the contactresistance of the p electrode. A small amount of oxygen may also beadded to the p-type layer being grown to remove the hydrogen remainingin the p-type layer, because the hydrogen can interfere with theactivation of Mg serving as the p-type impurity.

[0100] After p-type GaN contact layer 106 was grown, the atmosphere inthe reactor of the MOCVD apparatus was replaced by absolute nitrogencarrier gas and NH₃ and the temperature was decreased at a rate of 60°C./minute. After the substrate temperature reached 800° C., the NH₃supply was stopped and the substrate was allowed to stand at 800° C. forfive minutes and its temperature was then lowered to room temperature.In this process, the substrate may preferably be held at a temperatureof 650° C. to 900° C. and allowed to stand for three to ten minutes. Thetemperature may also be reduced preferably at a rate of not less than30° C./minute.

[0101] The grown film was evaluated by Raman spectroscopy and it wasfound that the film already had p-type characteristics (i.e., Mg wasalready activated) without annealing, a conventional technique formaking nitride semiconductor films have p-type conductivity. The contactresistance had already been reduced enough for forming the p electrode.When the conventional annealing to give p-type conductivity was alsoused, the rate of activated Mg was preferably improved.

[0102] Then the epi-wafer was taken out from the MOCVD apparatus andelectrodes were formed. Since n-type GaN substrate 100 was used, Hf andAu metal films were deposited on the back surface of substrate 100 inthis order to form n electrode 108. The n electrode materials may bereplaced by Ti/Al, Ti/Mo, Hf/Al or the like. In particular, Hf ispreferably used to reduce the contact resistance of the n electrode.

[0103] In forming the p electrode, Pd of 7 nm thick was vapor-depositedfor transparent electrode 106 and Au was vapor-deposited for p electrode107. Alternatively the material for the transparent electrode may be Nior Pd/Mo, Pd/Pt, Pd/Au, Ni/Au or the like.

[0104] Finally, a scriber was used to divide the product into chips. Indoing so the scriber was applied on the back surface of n-type GaNsubstrate 100 (the side having n electrode 108 deposited thereon) toprevent debris from adhering, in the scribing step, to the transparentelectrode side for taking light. In the scribing step, the product wasdivided into chips in such a manner that at least one side of the chiphas a cleavage plane of the nitride semiconductor substrate. Thisprevents the chips from having an abnormal geometry due to chipping,cracking and the like and thus increases yield per wafer.

[0105] In the above process, the light emitting nitride semiconductordiode device as shown in FIG. 1 was prepared, with different amounts ofdopant Si in the light emitting layer. The obtained device was examinedfor the emission intensity and the crystal system separation degree inthe light emitting layer and the relationship as shown in FIG. 8 wasobtained. When the light emitting layer was doped with the impurity (Si)in a concentration of 1×10¹⁸/cm³, 5×10¹⁸/cm³, 2×10¹⁹/cm³, or 1×10²⁰/cm³,the device had significantly high emission intensity. Preferable resultswere obtained in the impurity concentration range from 1×10¹⁷ to5×10²⁰/cm³.

[0106] In the device as shown in FIG. 1, the low temperature GaN bufferlayer may be replaced with a low temperature Al_(x)Ga_(1-x)N bufferlayer, wherein 0≦x≦1. Alternatively, the low temperature GaN bufferlayer may not be used. If the GaN substrate does not have a preferablesurface morphology, however, the low temperature Al_(x)Ga_(1-x)N bufferlayer, wherein 0≦x≦1, is preferably provided to improve the surfacemorphology. Herein the “low temperature buffer layer” refers to a bufferlayer grown at a temperature of approximately 450 to 600° C. The bufferlayer grown in such a temperature range is polycrystalline or amorphous.

[0107] In the device shown FIG. 1, the light emitting layer may have amulti-quantum well structure in place of the single quantum wellstructure. If the layer has a multi-quantum well structure, it may havea structure starting and ending with a barrier layer or a structurestarting and ending with a well layer. Not more than 10 well layerspreferably gave high intensity of the light emitting diode.

[0108] In the device shown in FIG. 1, the p-type Al_(0.1)Ga_(0.9)Ncarrier block layer may be replaced with an AlGaN layer having an Alcontent other than 0.1. An increased Al content can preferably enhancethe carrier confinement in the well layer. In contrast, the Al contentmay be reduced within a certain range that the carrier confinement ismaintained. In such a case, the carrier mobility in the carrier blocklayer can preferably be increased and the electrical resistance canpreferably be lowered. The carrier block layer is not limited to the3-element mixed crystal of AlGaN and it may be a 4-element mixed crystalsuch as AlInGaN, AlGaNP, AlGaNAs or the like.

[0109] In the device according to the present invention, the n electrodemay be formed on the n-type GaN layer exposed on the side of the pelectrode by dry etching, as shown in FIG. 4.

[0110] As regards the crystal plane on which the device structure shouldbe formed, the GaN substrate C (0001) plane may be replaced by the C(000-1) plane, the A {11-20} plane, the R {1-102} plane, the M {1-100}or {1-101} plane. Furthermore, a substrate surface forming an offsetangle within two degrees with the above crystal plane can preferablyhave a good surface morphology. In the present invention, any substratemade of nitride semiconductor, particularly including anAl_(x)Ga_(y)In_(z)N substrate, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1 and x+y+z=1,may be used. The substrate may be doped with Si, O, Cl, S, C, Ge, Zn,Cd, Mg or Be. For an n-type nitride semiconductor substrate, Si, O andCl are particularly preferable.

[0111] While the present device is produced by the MOCVD, it may beproduced by molecular beam epitaxy (MBE), hydride vapor phase epitaxy(HVPE) or the like.

EXAMPLE 2

[0112] A light emitting nitride semiconductor diode was fabricated as inExample 1 except that the GaN substrate 100 shown in FIG. 1 was replacedby a quasi GaN substrate 200 shown in FIG. 2 or a quasi GaN substrate200 a shown in FIG. 3(b) and that, as shown in FIG. 4, the n electrodewas formed on the same side as the p electrode. The quasi GaN substratewill be described with reference to FIGS. 2 and 3 and then will bedescribed a light emitting diode employing the quasi GaN substrate.

[0113] The quasi GaN substrate 200 shown in FIG. 2 is composed of a seedsubstrate 201, a low temperature buffer layer 202, an n-type GaN film203, an anti-growth film 204, and an n-type GaN thick film 205. QuasiGaN substrate 200 has seed substrate 201 other than a nitridesemiconductor substrate and seed substrate 201 is used as a base forgrowing n-type GaN thick film 205. The anti-growth film refers to a filmthat inhibits nitride semiconductor crystal from being grown directlythereon.

[0114]FIG. 3(a) shows an intermediate step in a process of producingquasi GaN substrate 200 a and FIG. 3(b) shows complete quasi GaNsubstrate 200 a. The quasi GaN substrate 200 a shown in FIG. 3(b) iscomposed of a seed substrate 201, a low temperature buffer layer 202, afirst n-type GaN film 203 a and a second n-type GaN film 203 b. As shownin FIG. 3(a), initially on seed substrate 201, low temperature bufferlayer 202 is formed and thereon the first n-type GaN film 203 a isformed and then the surface of GaN film 203 a is dry-etched orwet-etched to have a groove. The product is then transported again tothe crystal growth apparatus and the second n-type GaN film 203 b isdeposited to complete quasi GaN substrate 200 a (FIG. 3(b)). While, asshown in FIG. 3(a), the substrate has a groove only reaching anintermediate portion of the first n-type GaN film 203 a, it may have agroove reaching low temperature buffer layer 202 or seed substrate 201.

[0115] When a nitride semiconductor film was grown on quasi GaNsubstrate 200 or 200 a, the obtained film had a dislocation density (athreading dislocation density of approximately 3×10⁷/cm² and an etch pitdensity of approximately 7×10⁷/cm²), which is lower than that of thefilm grown on a sapphire substrate, a SiC substrate or the like (athreading dislocation density of approximately 1 to 10×10⁹/cm² and anetch pit density of approximately 4×10⁸/cm²).

[0116] The quasi GaN substrate shown in FIG. 2 has a higher threadingdislocation density at a portion 206 located exactly above the center ofan anti-growth film having a predetermined width and at a portion 207located exactly above the center of the anti-growth film free portionhaving a predetermined width. Similarly, the quasi GaN substrate shownin FIG. 3(b) has a higher threading dislocation density at a portion 208located exactly above the center of the groove having a predeterminedwidth and at a portion 209 located exactly above the center of thegroove free portion (plateau portion) having a predetermined width. Incontrast, in FIG. 2 a portion located at or near the center betweenportions 206 and 207 has a lowest threading dislocation density and sodoes a portion shown in FIG. 3 that is located at or near the centerbetween portions 208 and 209. Thus the quasi GaN substrate has a portionwith a high threading dislocation density and a portion with a lowthreading dislocation density in a mixed manner. Therefore, the quasisubstrate is inferior in device yield to the GaN substrate. It isrecommendable that a light emitting device should be fabricated at a lowthreading dislocation density region of the quasi GaN substrate.

[0117] Specific examples of seed substrate 200 include C-plane sapphire,M-plane sapphire, A-plane sapphire, R-plane sapphire, GaAs, ZnO, MgO,spinel, Ge, Si, 6H-SiC, 4H-SiC, 3C-SiC and the like. Specific examplesof anti-growth film 204 can include dielectric films such as SiO₂ film,SiN_(x) film, TiO₂ film and Al₂O₃ film, and metal films such as tungstenfilm. Alternatively, a hollow may be provided in place of theanti-growth film.

[0118] If the seed substrate is a conductive substrate of SiC, Si or thelike, the n electrode may be formed on the back surface of the substrateas shown in the FIG. 1. In this case, however, a high temperature bufferlayer should be substituted for the low temperature buffer layer. Thehigh temperature buffer layer refers to a buffer layer grown at notlower than 900° C. It should also contain at least Al, otherwise thenitride semiconductor film with good crystallinity cannot be formed onthe SiC or Si substrate. A most preferable material for the hightemperature buffer layer is AlN.

[0119] The crystal plane of the main surface of the seed may typicallybe the C {0001} plane, the A {11-20} plane, the R {1-102} plane, the M{1-100} plane, or the {l-101} plane. The substrate surface maypreferably form an offset angle within two degrees with the abovecrystal planes to have a good surface morphology.

[0120] The quasi GaN substrate was used to fabricate a light emittingdiode as shown in FIGS. 4 and 5. FIG. 4 is a cross section of the lightemitting diode and FIG. 5 is a top view thereof. As shown in FIG. 4, thelight emitting diode is composed of a substrate 300, a low temperatureGaN buffer layer 101 (of 50 nm in thickness), an n-type GaN layer 102, alight emitting layer 103, a p-type Al_(0.1)Ga_(0.9)N carrier block layer104, a p-type GaN contact layer 105, a transparent electrode 106, a pelectrode 107, an n electrode 108, and a dielectric film 109. In thisdiode, substrate 300 has the structure of the quasi GaN substrate 200 inFIG. 2 or that of the quasi GaN substrate 200 a in FIG. 3.

[0121] The light emitting diode is fabricated in such a manner that atleast the portions 206 and 207 in FIG. 2 or the portions 208 and 209 inFIG. 3 are excluded from the diode structure. Preferably, the formationof the light emitting diode starts at a position 1 μm distant in thelateral direction from each centerline of portions 206 and 207 or eachcenterline of portions 208 and 209. At the portions less than 1 μmdistant from each centerline, the threading dislocation density can berelatively high and cracks can readily be caused.

[0122] In the device shown in FIG. 4, the low temperature buffer layermay be a low temperature Al_(x)Ga_(1-x)N buffer layer, wherein 0≦x≦1.Alternatively, the low temperature GaN buffer layer may be omitted. Ifthe quasi GaN substrate does not have a preferable surface morphology,however, the low temperature Al_(x)Ga_(1-x)N buffer layer, wherein0≦x≦1, is preferably provided to improve the surface morphology.

[0123] Alternatively, seed substrate 201 may be removed from quasi GaNsubstrate 200 or 200 a by means of a grinder and the obtained substratemay be used as substrate 300 to fabricate the light emitting device asshown in FIG. 4. Alternatively, low temperature buffer layer 202 and theunderlying layer(s) may all be removed from quasi GaN substrate 200 or200 a by means of a grinder and the light emitting device may similarlybe fabricated. Alternatively, anti-growth film 204 and the underlyinglayer(s) may all be removed from quasi GaN substrate 200 or 200 a bymeans of a grinder and the light emitting device may similarly befabricated. When seed substrate 201 is removed, n electrode 111 can beformed on the back surface of the substrate as shown in the FIG. 1.Alternatively, seed substrate 200 may be removed after the lightemitting device is completed.

[0124] The crystal system separation that can be caused by doping thelight emitting layer with the impurity can efficiently be inhibited inthe light emitting device formed on the quasi GaN substrate as shown inFIG. 4. The light emitting device of this example also exhibitedcharacteristics similar to those obtained in Example 1.

EXAMPLE 3

[0125] A nitride semiconductor laser diode was fabricated, as shown inFIG. 6. The laser diode shown in FIG. 6 is composed of a C plane (0001),n-type GaN substrate 400, a low temperature GaN buffer layer 401, ann-type Al_(0.05)Ga_(0.95)N layer 402, an n-type In_(0.07)Ga_(0.93)Nanti-crack layer 403, an n-type Al_(0.1)Ga_(0.9)N clad layer 404, ann-type GaN optical guide layer 405, a light emitting layer 406, a p-typeAl_(0.2)Ga_(0.8)N carrier block layer 407, a p-type GaN optical guidelayer 408, a p-type Al_(0.1)Ga_(0.9)N clad layer 409, a p-type GaNcontact layer 410, an n electrode 411, a p electrode 412, and a SiO₂dielectric film 413.

[0126] While SiH₄ was adding to both of the barrier and well layers (ina Si concentration of 1×10¹⁸/cm³), 4 nm-thickIn_(0.05)Ga_(0.95)N_(0.98)P_(0.02) well layer and 6 nm-thickIn_(0.05)Ga_(0.95)N barrier layer were grown in three cycles in theorder of barrier/well/barrier/well/barrier/well/barrier to form lightemitting layer 406 of a multi-quantum well structure. The crystal systemseparation that can restrained in the obtained semiconductor laser. Thesemiconductor laser of this example exhibited a low threshold currentdensity.

[0127] Low temperature GaN buffer layer 401 may be replaced with a lowtemperature Al_(x)Ga_(1-x)N buffer layer, wherein 0≦x≦1. Alternatively,the low temperature GaN buffer layer may be omitted. If the GaNsubstrate does not have a preferable surface morphology, the lowtemperature Al_(x)Ga_(1-x)N buffer layer, wherein 0≦x≦1, is preferablyprovided to improve the surface morphology.

[0128] In_(0.07)Ga_(0.93)N anti-crack layer 403 may be replaced withanother InGaN layer having an In content of other than 0.07.Alternatively, the InGaN anti-crack layer may be omitted. If there is asignificant lattice mismatch between the clad layer and the GaNsubstrate, InGaN anti-crack layer should be provided.

[0129] The structure of the light emitting layer starting and endingwith a barrier layer may be replaced by the structure starting andending with a well layer. The number of the well layers is not limitedto three, and ten or less well layers were able to provide a lowthreshold current density and to generate continuous oscillation at roomtemperature. In particular, the devices having two to six well layerspreferably had a low threshold current density.

[0130] In the process of forming the multi-quantum well structure of thelight emitting layer, the 4 nm-thick In_(0.05)Ga_(0.95)N_(0.98)P_(0.02)well layer and 6 nm-thick In_(0.05)Ga_(0.95)N barrier layer may bereplaced with different material layers (see the section entitledStructure of Light Emitting Layer). The well layer and the barrier layermay have a thickness of 0.4 nm to 20 nm and a thickness of 1 nm to 20 nmrespectively, so that they can have good crystallinity to achieve theeffect of the present invention sufficiently.

[0131] Since the barrier layer in this example does not contain any ofAs, P or Sb, the barrier layer may be free of the impurity.Alternatively, as far as the above-described requirements for theimpurity are satisfied, an impurity other than Si may be used, or thedose of the impurity may be changed.

[0132] P-type Al_(0.2)Ga_(0.8)N carrier block layer 407 may be replacedwith an AlGaN layer having an Al content of other then 0.2.Alternatively, the carrier block layer may be omitted. However, thecarrier block layer was able to contribute to a lower threshold currentdensity. The carrier block layer can serve to confine the carriers inthe light emitting layer. A higher Al content in the carrier block layercan preferably enhance the carrier confinement. On the other hand, theAl content may be reduced within a certain range that the carrierconfinement is maintained. In such a case, the carrier mobility in thecarrier block layer can preferably be increased and a low electricalresistance can preferably be obtained in the device.

[0133] As for the p- and n-type clad layers, Al_(0.1)Ga_(0.9)N may bereplaced with another 3-element crystal of AlGaN having an Al content ofother than 0.1. A higher mixing ratio of Al can provide a larger energygap and a larger difference of index of refraction between the cladlayer and the light emitting layer. In such a case, the carriers and thelight can efficiently be confined in the light emitting layer so thatthe lasing threshold current density can be reduced. The Al content maybe reduced within a certain range that the carrier and light confinementis maintained. In such a case, the carrier mobility in the clad layercan preferably be increased and a low electrical resistance canpreferably be obtained in the laser device, so that a low operatingvoltage can be achieved in the device.

[0134] Preferably, the AlGaN clad layer has a thickness of 0.7 μm to 1.5μm. The clad layer with such a thickness can provide the laser devisewith a good unimodal, vertical transverse mode and an increased lightconfinement efficiency, so that the optical characteristics of the lasercan be improved and the lasing threshold current density can be reduced.

[0135] The clad layer is not limited to the 3-element mixed crystal ofAlGaN and it may be a 4-element mixed crystal such as AlInGaN, AlGaNP orAlGaNAs. Alternatively, the p-type clad layer may have a superlatticestructure composed of a p-type AlGaN layer and a p-type GaN layer, or ofa p-type AlGaN layer and a p-type InGaN layer to reduce the deviceresistance.

[0136] In this example, the effect of the C {0001} plane GaN substratewas similar to that in Example 1. On the other hand, the effect of thequasi GaN substrate in place of the GaN substrate was similar to that inExample 2. If the quasi GaN substrate is used, the ridged stripe asshown in FIG. 6 is preferably formed at a position separate from theportions 206 and 207 in FIG. 2 or separate from the portions 208 and 209in FIG. 3. More preferably, the ridged stripe is formed at a portion 1μm distant, in the lateral direction, from each centerline of portions206 and 207 or from each centerline of portions 208 and 209. Theportions less than 1 μm distant in the lateral direction from eachcenterline can have a high threading dislocation density and can beliable to cause cracks.

EXAMPLE 4

[0137] A device was fabricated, as in Examples 1 to 3, except that thelight emitting layer was doped with carbon (C) impurity in aconcentration of 1×10²⁰/cm³. Similar characteristics were obtained.

EXAMPLE 5

[0138] A device was fabricated, as in Examples 1 to 3, except that thelight emitting layer was doped with Mg impurity in a concentration of1×10¹⁷/cm³. Similar characteristics were obtained.

[0139] Light Emitting Apparatus

[0140] The light emitting nitride semiconductor diode of the presentinvention can be used to provide a light emitting apparatus, such as adisplay device, white-light source device or the like. For example, thelight emitting diode of the present invention can be employed for atleast one of the three primary colors of light, i.e., red, green andblue to provide a display devise.

[0141] The amber-color light emitting diode employing a conventionalInGaN well layer is not marketable for its poor reliability and lowemission intensity. The In content in the conventional InGaN well layeris so high that significant composition separation can be caused by In(i.e., a high In content portion and a low In content portion can beformed). On the other hand, As, P or Sb contained in the light emittinglayer can serve to reduce the band gap energy of the light emittinglayer (the well layer), like In. Therefore, In can be reduced or omittedby the addition of As, P or Sb to the light emitting layer (the welllayer). The conventional nitride semiconductor layer containing at leastone of As, P and Sb, however, has crystal system separation as describedabove, and its crystallinity degraded can result in a low emissionintensity. Thus the conventional device cannot derive substantialadvantage from As, P or Sb. The crystal system separation and the likecan also disturb the interface between the well and barrier layers. Thusthe multi-quantum well structure can hardly be fabricated or the lightemitting device can have increased unevenness of color or decreasedemission intensity.

[0142] In the present invention, the impurity added to the lightemitting nitride semiconductor layer containing at least one of As, Pand Sb can reduce the crystal system separation to overcome theabove-described disadvantages. According to the present invention, thecrystallinity of the light emitting layer can be improved and the lightemitting diode can derive the advantage from As, P or Sb contained inthe light emitting layer. The light emitting device according to thepresent invention can have any emission wavelength in the range of 360nm to 650 nm. The wavelength and the composition of the light emittinglayer are exemplarily presented in the above section entitled Structureof Light Emitting Layer.

[0143] According to the present invention, light emitting diodes of thethree primary colors can be combined together to provide a white-lightsource device. Alternatively, the light emitting diode of the presentinvention having an emission wavelength from the w range to theviolet-color range (from 360 nm to 420 nm) may have fluorescent paintapplied thereon to provide the white-light source device. Suchwhite-light sources can replace a halogen light source for conventionalliquid crystal displays and serve as a backlight with low powerconsumption and high intensity for the displays. It can be used as abacklight for a liquid crystal display allowing man-machine interfacevia mobile notebook personal computers, cellular phones and the like. Itcan provide a miniaturized and clear liquid crystal display.

[0144] Optical Pickup Device

[0145] The nitride semiconductor laser of the present invention isapplicable to optical pickup devices.

[0146] The light emitting nitride semiconductor layer according to thepresent invention contains at least one of As, P and Sb. These elementscontained in the light emitting layer can reduce the effective mass ofthe electrons and the holes in the light emitting layer, and increasethe mobility of the electrons and holes. The former suggests that thecarrier population inversion for lasing can be generated by a smallercurrent injection. The latter suggests that if the electrons and holesare consumed by the radiative recombination in the light emitting layer,new electrons and holes can rapidly be injected through diffusion. Thusit was believed that the nitride semiconductor laser with As, P or Sbcould have a lower threshold current density and superiorself-oscillation characteristics (or lower noise characteristics) ascompared with the nitride semiconductor laser completely free of As, Pand Sb. If the crystal system separation occurs in the As, P or Sbcontaining light emitting layer of the nitride semiconductor device,however, it will be difficult to obtain such advantages.

[0147] In the present invention, the impurity added to the lightemitting nitride semiconductor layer can reduce the crystal systemseparation. According to the present invention, the light emitting layercan improve in crystallinity and the semiconductor laser can have a lowthreshold current density accompanied by higher output, and a long life.According to the present invention, a semiconductor laser havingsuperior noise characteristics can be fabricated. For example, a nitridesemiconductor laser of the present invention having an oscillationwavelength of 380 to 420 nm can have a lower lasing threshold currentdensity, a smaller amount of spontaneous emission light in the laserlight, and less susceptible to noise as compared with a conventionalInGaN-based nitride semiconductor laser. The present semiconductor lasercan reliably work under a high power (e.g. 50 mW) and a high-temperatureambient. Such a laser is suitable for an optical disc for high densityrecording and reproduction.

[0148]FIG. 7 shows an optical disc device employing the nitridesemiconductor laser diode device according to the present invention. Inthe optical disc device, the nitride semiconductor laser emits laserbeam, which is transmitted via an optical modulator, a splitter, afollow-up mirror and a lens to illuminate an optical disc. The beam fromthe splitter is detected by a photodetector. The photodetector outputs asignal which is in turn transmitted to a control circuit. The controlcircuit sends signals to a motor actuating the disc, the semiconductorlaser, the optical modulator and the follow-up mirror, respectively. Thelaser beam is modulated by the optical modulator in response toinformation input and recorded on the disc via the lens. Inreproduction, laser beam optically changed by pit arrangement on thedisc is transmitted though the splitter and detected by thephotodetector to form a reproduced signal. This series of operations arecontrolled by the control circuit. Normally, a laser output ofapproximately 30 mW is provided in recording and that of approximately 5mW is provided in reproduction.

[0149] Besides the optical disc device as described above, the deviceaccording to the present invention is also applicable to laser printers,barcode readers, and projectors using three primary color (blue, red,green) laser diodes.

[0150] In the present invention, the impurity added to the lightemitting layer can reduce the crystal system separation in the lightemitting layer. According to the present invention, the light emittingnitride semiconductor device with a high luminous efficacy can beprovided. According to the present invention, such a device is appliedto a light emitting apparatus and optical pickup device.

[0151] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. A light emitting nitride semiconductor device,comprising: a substrate made of nitride semiconductor crystal or asubstrate having a nitride semiconductor crystal film grown on acrystalline material other than said nitride semiconductor crystal; ann-type layer and a p-type layer made of nitride semiconductor and formedon said substrate; and a light emitting layer provided between said n-and p-type layers, wherein said light emitting layer is formed of a welllayer or a combination of well and barrier layers; of said layer orlayers forming said light emitting layer, at least said well layer ismade of a nitride semiconductor containing an element X, N and Ga, saidelement X being at least one selected from the group consisting of As, Pand Sb; in said nitride semiconductor forming said light emitting layer,the ratio of the number of the atoms of said element X to the sum ofsaid number of the atoms of said element X and the number of the atomsof said N, is not more than 30 atomic percent; and of said layer orlayers forming said light emitting layer, at least said well layercontains as an impurity at least one element selected from the groupconsisting of Mg, Be, Zn, Cd, C, Si, Ge, Sn, O, S, Se and Te.
 2. Thelight emitting nitride semiconductor device of claim 1, wherein saidimpurity is contained so as to improve crystallinity of said well layer.3. The light emitting nitride semiconductor device of claim 1, wherein atotal content of said impurity is 1×10¹⁷ to 5×10²⁰/cm³.
 4. The lightemitting nitride semiconductor device of claim 1, wherein said nitridesemiconductor crystal or said nitride semiconductor crystal film of saidsubstrate or said light emitting nitride semiconductor device has athreading dislocation density of not more than 3×10⁷/cm² or an etch pitdensity of not more than 7×10⁷/cm².
 5. The light emitting nitridesemiconductor device of claim 3, wherein said nitride semiconductorcrystal or said nitride semiconductor crystal film of said substrate orsaid light emitting nitride semiconductor device has a threadingdislocation density of not more than 3×10⁷/cm² or an etch pit density ofnot more than 7×10⁷/cm².
 6. The light emitting nitride semiconductordevice of claim 1, wherein said light emitting layer is a multi-quantumwell layer.
 7. The light emitting nitride semiconductor device of claim3, wherein said light emitting layer is a multi-quantum well layer. 8.The light emitting nitride semiconductor device of claim 4, wherein saidlight emitting layer is a multi-quantum well layer.
 9. A light emittingapparatus, comprising said light emitting nitride semiconductor deviceas recited in claim 1 and having an emission wavelength of 360 nm to 650nm.
 10. An optical pickup apparatus, comprising a light emissionapparatus comprising said light emitting nitride semiconductor device asrecited in claim 1 and having an oscillation wavelength of 360 nm to 420nm.